Photo detecting in the near infrared regime, i.e., having a wavelength of between 0.7 μm to 2 μm, has many applications, such as in fiber-optic communication, security applications, machine vision and night vision imaging. Although III-V compound semiconductors provide superior optical performance over their silicon-based counterparts, the compatibility of silicon based materials with current silicon-IC technology provides the possibility of making cheap, small and highly integrated optical systems. The following references provide additional background for the invention: Maiti et al., Strained Silicon Heterostructures: Materials and Devices, Chapter 10: Si/SiGe Optoelectronics, The Institution of Electrical Engineer, 2001; Murtaza et al., Room Temperature Electroabsorption in GexSi1-x PIN Photodiode, IEEE Trans. on Electron Devices, 2297-2300, Vol. 41, No. 12, 1994; Tashiro et al., A Selective Epitaxial SiGe/Si Planar Photodetector for Si-Based OEICs, IEEE Trans. on Electron Devices, 545-550, Vol. 44, No. 4, 1997; Vonsovici et al., Room Temperature Photocurrent Spectroscopy of SiGe/Si p-i-n Photodiodes Grown by Selective Epitaxy, IEEE Trans. on Electron Devices, 538-542, Vol. 45, No. 2, 1998; and Jones et al., Fabrication and Modeling of Gigahertz Photodetectors in Heteroepitaxial Ge-on-Si using Graded Buffer Layer Deposited by Low Energy Plasma Enhanced CVD, IEDM, 2002.
Silicon photodiodes are widely used as photodetectors for visible light due to their low dark current and compatibility with silicon IC technologies. The use of Si1-xGex (SiGe) alloys in silicon processing permits photo detection operating in the 0.8 μm to 1.6 μm wavelength regime.
SiGe alloys have larger lattice constants than pure silicon, thus, the epitaxial growth of SiGe on silicon has a critical thickness, above which the film begins to relax by the nucleation of dislocations. The critical thickness of SiGe depends on the germanium concentration and device process temperature. Houghton, Strain relaxation kinetics in Si1-xGex/Si heterostructures, J. Appl. Phys. Vol. 780, No. 4, 1991.
A high germanium concentration and high device process temperature result in a smaller critical thickness. In common practice, the SiGe critical thickness is in the range of few hundred angstroms to a maximum of a couple thousand angstroms. Once the SiGe thickness is grown above its critical thickness, lattice defects in SiGe are inevitable. An IR photo detector built on SiGe containing lattice defects will have a high dark current and produce electronic noise.
Quantum efficiency is the number of electron-hole pairs generated per incident photon and is a parameter for photo detector sensitivity. The quantum efficiency is defined as:η=(Ip/q)/(Popt/hν)  (1)where Ip is the photo-generated current by the absorption of incident optical power Popt at the light frequency ν, where q is the electron charge, and h is Planck's constant.
One of the key factors that determines the quantum efficiency is the absorption coefficient, α. Silicon has a cutoff wavelength of about 1.1 μm and is transparent to wavelengths beyond ˜1.2 μm. The SiGe absorption edge shifts to the red with increasing germanium mole fraction and is shown in FIG. 1. The absorption coefficient of SiGe alloy is small and the critical thickness limits the absorbing layer thickness. The major aim of SiGe based photo detectors is to achieve high quantum efficiency and integration with existing silicon electronics.
One way to increase the optical path and improve the quantum efficiency is to illuminate the edge of the photo detector with light so that the light propagates parallel to the heterojunction (SiGe/Si) interfaces. However, this does not allow the device to be used in image detection. Growing strained, defect-free SiGe films or SiGe/Si multilayer structures on the sidewalls of etched silicon structures has been disclosed by Lee et al., Surface-Normal Optical Path Structure for Infrared Photodetection, U.S. Patent Publication No. 2005/0136637-A1, published Jun. 23, 2005; and Tweet et al., Vertical Optical Path Structure for Infrared Photodetection, U.S. Patent Publication No. 2005/0153474-A1, published Jul. 14, 2005. In devices incorporating the technology described in the two preceding references, illumination of the device is normal to the silicon substrate, however, light travels parallel to the heterojunction interface to increase the optical path length. Therefore, two-dimensional IR image detection may be achieved within thin SiGe or SiGe/Si film thicknesses.
Fabrication of high quality, defect-free strained SiGe films requires SiGe growth on a defect-free silicon surface. However, referring to FIG. 2, reactive ion-etching (RIE) of silicon usually results in a sloped sidewall, shown generally at 10, and in poor crystal quality near the sidewall surface, as shown generally at 12, where a damaged area of silicon is formed near an RIE-etched sidewall. Also, the surface is often rather rough. Ideally, these defects may be cured by use of a selective etch, which etches the desired sidewall crystal plane more slowly than it does other planes. For various kinds of silicon device fabrication, silicon substrates having a (001) plane parallel to the wafer surface are most commonly used. Etching of a trench in this substrate renders the sidewalls parallel to the (110) planes or (100) planes, depending on the azimuthal rotation of the wafer. Making these sidewalls more vertical requires a selective etch, which etches the (110) or (100) planes, respectively, more slowly than other planes. However, such an etch process is not known in the prior art. Instead, there are well-known selective etches which etch the (111) plane much more slowly than other planes.
One way around this problem is to change the substrate to one with the silicon (110) plane parallel to the wafer surface, Liu et al., Multi-Fin Double-Gate MOSFET Fabricated by using (110)-Oriented SOI wafers and Orientation-Dependent Etching, Electrochemical Society Proceedings, vol. 2003-06, 566 (2003). Then, the sidewall plane may be silicon (111) or any other related, equivalent orthogonal plane. Liu et al. used a 2.38% tetramethylammonium hydroxide (TMAH) solution to etch vertical sidewalls and form a silicon fin structure with rectangular cross-section on silicon-on-insulator wafers. These were then used to fabricate silicon-FINFET (FIN Field-Effect Transistor) devices. Liu et al. report that TMAH etches (110) planes 23-25 times faster than silicon (111) planes. In addition, the use of a selective wet etch instead of RIE results in undamaged crystalline silicon at the sidewall surfaces.